Trialkoxysilanes, especially trimethoxysilane, triethoxysilane and tri(isopropoxy)silane, are used in the production of silane coupling agents. One method of synthesis of trialkoxysilanes is directly from silicon and an alcohol in the presence of copper or a copper compound. This method is known variously in the art as the Direct Synthesis, the Direct Reaction, the Direct Process or the Rochow Reaction. For trialkoxysilanes, it is most conveniently performed in slurry reactors.
In a slurry reactor for the Direct Synthesis of trialkoxysilane, catalytically-activated silicon particles are maintained in suspension in a thermally stable, high boiling solvent and are made to react with an alcohol at an elevated temperature. The product stream exiting the reaction zone comprises a mixture of unreacted alcohol, trialkoxysilane, tetraalkoxysilane, alkyldialkoxysilane, alkyltrialkoxysilane and condensed silicates. Trialkoxysilane is usually the desired product. However, there are instances in which the tetraalkoxysilanes and alkyldialkoxysilanes are also sought and methods and processes which increase the formation of these co-products are known in the art. The trialkoxysilane product and desired co-products are typically recovered by distillation.
The rate of the Direct Synthesis of a trialkoxysilane is the temporal consumption of the raw materials (that is, alcohol or silicon), or the temporal formation of products (trialkoxysilane,optionally including by-products). Familiar units are weight percent silicon conversion per hour, or kilograms product per kilogram silicon per hour.
Selectivity is the preference for the trialkoxysilane under the reaction conditions. It is expressed herein as the gravimetric ratio trialkoxysilane/tetraalkoxysilane. Alternatively, selectivity can be expressed as mole percentage, i.e., 100 moles trialkoxysilane/molar sum of all silicon-containing products.
Stability is the maintenance of desirable rate and selectivity until all raw materials are consumed, or consumed beyond a preset criterion. The progress of the Direct Synthesis can be monitored by determining product composition and/or reaction rate as a function of time or silicon conversion. In general, the reaction profile shows an initial period (referred to as the induction period) of increasing rate and increasing trialkoxysilane concentration in the reaction mixture, after which the reaction settles into a steady state. In this state, the composition of the reaction mixture remains approximately constant. A period of declining rate and decreasing trialkoxysilane content in the product mixture follows the steady state.
It is generally accepted that the actual catalysts in the Direct Synthesis of silicon are the copper-silicon alloys or intermetallics and solid solutions formed by the diffusion of copper into silicon, or by the reaction of copper compounds with silicon. Thus, the copper-containing raw materials effective in activating silicon for the Direct Synthesis with alcohols are all catalyst precursors and will be referred to as such. A wide variety of such precursors has been disclosed in the prior art.
U.S. Pat. No. 3,641,077 discloses the slurry-phase preparation of trialkoxysilanes by directly reacting copper-silicon mass, suspended in silicone oil, with alcohol at 250-300° C. The copper-silicon mass contains about 10 weight percent copper and is prepared by heating copper and silicon above 1000° C. in a furnace in a stream of hydrogen gas.
U.S. Pat. No. 3,775,457 discloses the use of cuprous chloride and HF or HCl in the slurry-phase Direct Synthesis of silicon and alcohols. Ammonium chloride is used to restore waning reactivity.
Japanese Patent Publication 55-28929 (1980) acknowledges that activation of silicon with CuCl does not always lead to desirable rate and selectivity. The patent teaches treatment of CuCl with nitrites, naphthalenes, biphenyls and anthracenes prior to its use in the Direct Synthesis.
Japanese Patent Publication 55-2641 (1980) discloses the use of cyclic ethers such as dibenzo-18-crown-6 to improve the reaction rate and the yield of trialkoxysilane.
U.S. Pat. No. 5,362,897 discloses the use of specially prepared “wet process” CuCl, in preference to commercial “dry process” CuCl, and silicon containing 0.30-0.37 weight percent aluminum to obtain high reaction rates and silicon conversions. “Wet process” CuCl is defined (column 2, lines 51-54) as that “prepared through the steps of crystallization and separation and drying. Dry process CuCl is prepared from metallic copper and chlorine gas (column 2, lines 62-65).
U.S. Pat. No. 5,527,937 discloses a slurry-phase process for the Direct Synthesis of triethoxysilane wherein CuCl is the copper source, tri- and tetra-toluenes and/or their alkyl-substituted derivatives are the solvents and dimethylsilicone oils are employed as antifoaming agents. The method by which CuCl was prepared is not disclosed.
Japanese Patent Application 3-354055 (1991) discloses the use of copper alkoxides, with or without copper chlorides, as catalyst precursors for the Direct Synthesis of trialkoxysilanes.
Japanese Patent Application 6-306083 (1994) discloses improved selectivity and rate for the Direct Synthesis of trialkoxysilanes are realized when a an organosulfur compound such as a mercaptan, disulfide, thioheterocyclic (e.g.,thiophene) or and Cu(II) thio-complex (e.g., bis(4-methylthiophenol)copper(II) is present in the reaction slurry catalyzed with a copper alkoxide. The patent discloses that appropriate quantities of these compounds can be added to the reaction when selectivity has decreased. Alternatively, the compounds can be introduced at the outset of the reaction or continuously in the alcohol feed.
Japanese Patent Application 10-168084 (1998) and Japanese Patent Application 10-338696 (1998) both disclose cupric oxide having a water content of less than 3000 parts per million and an average particle size of 0.1-50 micrometers as the catalyst precursor in a process for manufacturing trialkoxysilanes.
U.S. Pat. No. 6,727,375 and Standke et al., Silicon for the Chemical Industry VI, pp. 225-231 (2002), disclose processes for the production of halogen-free triethoxysilane comprising the reaction of ethanol with HF-treated silicon in MARLOTHERM® S (a synthetic heat-transfer medium) in the presence of copper (II) neononanoate at 190-250° C. in a bubble column.
U.S. Pat. No. 6,380,414 discloses the use of freshly precipitated CuO with surface areas ≧10 square meters per gram as a copper source to catalyze the Direct Synthesis of trialkoxysilanes. U.S. Pat. No. 6,410,771 discloses that fluorinated copper salts are also effective for the same purpose.
JP 01 213287 discloses a slurry-phase method for manufacturing trialkoxysilanes characterized by the use of “auxiliary catalysts” such as metal halides, sulfates, nitrates and/or phosphates in addition to a source of catalytic copper. These “auxiliary catalysts” are used at 0.0001-0.2 mole per mole of copper catalyst, preferably 0.001-0.03 mole per mole of copper catalyst. NaH2PO4 and KH2PO4 are the only phosphates specified in the disclosure. The use of NaH2PO4 was illustrated at 1.8 weight percent relative to CuCl.
U.S. Pat. Nos. 6,580,000 and 6,680,399 disclose the use of copper (II) organophosphate salts and tetraalkyl orthosilicates for the Direct Synthesis of triethoxysilane. Copper (II) organophosphates, for example copper (II) bis(diethyl phosphate), are soluble in ethanol as well as in tetraethyl orthosilicate and its oligomers. The disclosed processes produce reaction mixtures with selectivity less than one up to about three.
U.S. Pat. No. 4,727,173 discloses that the use of copper (II) hydroxide avoids limitations associated with cuprous chloride and provides a high selectivity to trialkoxysilanes. U.S. Pat. Nos. 6,580,000, 6,680,399 and 6,727,375 dispute the improvements claimed for copper (II) hydroxide and present comparative data showing superior performance of copper (II) organophosphates and copper (II) neononanoate in the Direct Synthesis of triethoxysilane.
U.S. Pat. No. 5,728,858 discloses that when copper (II) hydroxide is used in combination with an alkylated benzene solvent such as dodecylbenzene, the Direct Synthesis of trialkoxysilane becomes unstable after approximately 25-35 weight percent of the silicon has been reacted. When methanol is the alcohol reactant, at temperatures above about 220° C., trimethoxysilane content in the reaction product declines from approximately 90-95 weight percent to approximately 50-60 weight percent but recovers to between 80-95 weight percent after about 60 percent silicon conversion. Simultaneously with this loss of selectivity is the increased formation of methane, water and dimethyl ether. Methane and dimethyl ether formation indicate an inefficient use of the methanol reactant.
Water reacts with trialkoxysilanes and tetraalkoxysilanes to produce soluble, gelled and/or resinous organic silicates. Formation of these silicates represents inefficiency in the Direct Synthesis process. Additionally, the silicates contribute to foaming and incomplete recovery of the reaction solvent as disclosed in U.S. Pat. Nos. 5,783,720 and 6,090,965.
U.S. Pat. No. 5,728,858 discloses the reductive activation of copper (II) hydroxide/silicon slurries with hydrogen gas, carbon monoxide, monosilane or polyaromatic hydrocarbons to obtain desirably active, selective and stable Direct Synthesis of trialkoxysilanes in alkylated benzene solvents such as NALKYLENE® 550BL. Reductive activation affords a steady-state region between about 10-70 weight percent silicon conversion, increased silicon conversion and increased selectivity to trimethoxysilane.
Published U.S. patent applications 2003/0032829 and 2003/0065204 disclose that nanosized copper catalyst precursors afford superior performance in the Direct Synthesis of silanes. The nanosized copper catalyst precursors are prepared from copper (II) hydroxide and other copper sources as described in published U.S. patent applications 2003/0051580 and 2004/0009117.
There appears to be disagreement in the prior art about the effectiveness of copper (II) hydroxide as a catalyst precursor. Additionally, in spite of the improvements and advances taught in the cited prior art, there is a need for higher stability, selectivity and rate, improved raw materials efficiency and controlled by-product formation and less waste generation in the Direct Synthesis of trialkoxysilanes. In particular, there is a need for a Direct Synthesis of trimethoxysilane which produces less than 10 weight percent, and preferably less than 6 weight percent, tetramethoxysilane. There is also a need for a Direct Synthesis in which the formation of useful by-products (also termed co-products) such as CH3SiH(OCH3)2 can be controllably increased.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the invention to provide a Direct Synthesis process for producing trialkoxysilane from silicon metal and methanol and higher alcohols with reduced co-production of tetraalkoxysilane.
In is another object of the invention to provide such an improved Direct Synthesis process comprising the use of organic and inorganic phosphates, phosphonates and phosphites as additives to increase and/or maintain reaction rate at desirable values and avoid or reduce deactivation, and increase silicon conversion, while maintaining selectivity at desirably high values.
Yet another object of the invention is to utilize the aforesaid phosphorus-containing additives as nanosize materials when they are in solid form.